Image sensors provide a grid of pixel unit cells for recording an intensity or brightness of incident light. In some applications, each pixel unit cell responds to the incident light by accumulating charge carriers (such as electrons and/or holes) generated when the incident light passes into/through a silicon layer. The greater intensity or brightness of the incident light, the more charge carriers are generated. The charge carriers are picked-up by a pick-up circuit and converted into an electrical signal which is subsequently usable by another circuit to provide color and/or brightness information for suitable applications, such as digital cameras. According to various types of light sensing elements and pick-up circuits, some image sensors are recognizable as a charge-coupled device (CCD), a complementary metal oxide semiconductor (CMOS) image sensor (CIS), an active-pixel sensor (APS), or a passive-pixel sensor.